1. Field of the Invention
This invention relates generally to near-field transducers (NFT) like those used in a thermal assisted recording (TAR) type of magnetic recording disk drive, and more particularly to a TAR disk drive with patterned media.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media, also called bit-patterned media (BPM), have been proposed to increase the data density. In patterned media, the magnetic material on the disk is patterned into small isolated data islands or islands arranged in concentric data tracks. Each island contains a single magnetic “bit” and is separated from neighboring islands by a nonmagnetic region. This is in contrast to conventional continuous media wherein a single “bit” is composed of multiple weakly-coupled neighboring magnetic grains that form a single magnetic domain and the bits are physically adjacent to one another. Patterned-media disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, or perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer. To produce the required magnetic isolation of the patterned islands, the magnetic moment of the regions between the islands must be destroyed or substantially reduced so as to render these regions essentially nonmagnetic. Alternatively, the media may be fabricated so that that there is no magnetic material in the regions between the islands.
In one type of patterned media, the data islands are elevated, spaced-apart pillars that extend above the disk substrate surface to define troughs or trenches on the substrate surface between the pillars. This type of patterned media is of interest because substrates with the pre-etched pattern of pillars and trenches can be produced with relatively low-cost, high volume processes such as lithography and nanoimprinting. The magnetic recording layer material is then deposited over the entire surface of the pre-etched substrate to cover both the ends of the pillars and the trenches. The trenches are recessed far enough from the read/write head to not adversely affect reading or writing. This type of patterned media is described by Moritz et al., “Patterned Media Made From Pre-Etched Wafers: A Promising Route Toward Ultrahigh-Density Magnetic Recording”, IEEE Transactions on Magnetics, Vol. 38, No. 4, July 2002, pp. 1731-1736.
Heat-assisted magnetic recording (HAMR), also called thermally-assisted recording (TAR), has been proposed. In TAR systems, an optical waveguide with a near-field transducer (NFT) directs heat from a radiation source, such as a laser, to heat localized regions of the magnetic recording layer on the disk. The radiation heats the magnetic material locally to near or above its Curie temperature to lower the coercivity enough for writing to occur by the write head. This type of TAR disk drive is described in U.S. Pat. No. 5,583,727 and U.S. Pat. No. 6,982,844. TAR systems have been proposed for patterned-media disk drives, wherein each data island is heated simultaneously with the application of the write field from the write head.
In conventional magnetic recording where the data bits are written on continuous media, there is no requirement to write to precise positions on the media since all of the media contains magnetic material. However, with patterned media, because the data islands are single-domain, the transitions between bits may occur only between islands. Since the magnetic transitions are restricted to predetermined locations governed by the locations of individual islands, it is necessary to synchronize the reversal of write current in the write head with the passing of individual islands under the head. U.S. Pat. No. 6,754,017 B2, assigned to the same assignee as this application, describes a magnetic recording disk drive with patterned media that uses a special pattern-sensor that senses the data islands before they pass beneath the write head and generates a write-clock signal. U.S. patent application Ser. No. 11/209,089, filed Sep. 11, 2008, published as US 2010/0061018 A1 and assigned to the same assignee as this application, describes a TAR disk drive with a radiation detector that detects radiation from the NFT that is reflected from the data islands and nonmagnetic regions as the disk rotates, with the radiation detector output being used to control the clocking of write pulses from the write head.
What is needed is a patterned-media TAR disk drive that has a method for detecting the data islands to generate an accurate write-clock signal so that the data can be accurately written to the patterned data islands.